As polishing material which accurately polishes in a production method of an optical glass or a semiconductor device, there are, diamonds, cerium oxide, iron oxide, aluminum oxide, zirconium oxide, colloidal silica, etc.
Some main constituent elements of the polishing material are obtained from minerals which are not found inside Japan, and some are resources relying on import. In addition, most are expensive material. Therefore, it is necessary to technically consider reuse of resources in liquid waste of polishing material including used polishing material.
As a method of processing waste water including suspended fine particles typically generated in various industrial fields, the suspended fine particles are aggregated and separated using a neutralizer, inorganic aggregating agent, macromolecular aggregating agent and the like, the processed water is discharged, and the aggregated and separated sludge is burned. With this, the waste water is disposed.
Polishing target components, such as waste from the optical glass, generated in large amounts in the polishing process is mixed in the liquid waste including used polishing material. Normally, it is difficult to effectively separate the polishing material component and the polishing target component included in the liquid waste. Therefore, at present, the polishing material liquid waste is often disposed after use. This causes the problems of cost in disposal.
Therefore, lately, it is becoming important to efficiently recover and reuse main constituent elements of the polishing material in order to conserve resources of highly scarce elements.
As a method of recovering the polishing material including the diamond abrasive, the following method is disclosed, the polishing slurry liquid waste is centrifuged, the metallic component is dissolved by processing with mineral acid such as nitric acid, and the liquid phase and the solid phase are separated to recover the diamond abrasive (for example, see patent document 1).
However, usually, few tens of liters to few hundreds of liters of polishing material slurry are repeatedly used in the polishing process. According to the method of patent document 1, the use of acid becomes a heavy burden to the environment.